This invention relates generally to electrical propulsion systems for diesel electric traction vehicles such as locomotives and, more particularly, to a self-test circuit for determining the operability of a shorted diode protection circuit in such a system.
In a conventional diesel electric traction vehicle such as a locomotive, a diesel engine is used to drive a synchronous generator that supplies electric current to a plurality of electric traction motors whose rotors are drivingly coupled to the respective wheel axle sets of a locomotive. The generator is typically a three-phase traction alternator having a rotor mechanically coupled to the output shaft of the diesel engine. When excitation current is supplied to field windings of the generator, alternating voltages are generated in the three-phase armature windings. These voltages are coupled to a full-wave bridge rectifier and converted to direct current power on a DC link. In a traction vehicle in which the electric motors are alternating current (AC) motors, the voltage on the DC link is applied to a plurality of inverters which convert the DC voltage to controlled frequency AC voltage for application to the motors.
An on-board control system controls operation of the diesel engine, synchronous generator and inverter in such a manner as to regulate the power applied to the traction motors so as to provide a desired amount of power at the wheels of the traction vehicles to attain a desired speed. In most systems, the diesel engine is operated at a constant speed and the power supplied by the synchronous generator is regulated by varying the field current. Variations in the field current at constant speed will control the magnitude of voltage produced at the armature windings. During normal running conditions, the current being generated by the synchronous generator may be several thousand amperes. If the rectifier circuit connected to the armature windings were to fail to a short circuit condition under such high currents, the amplitude of the armature current will tend to increase abruptly to a much higher peak than normal and then to decay with time. Such initial high current conditions may severely damage the generator and prevent operation of the traction vehicle. For that reason, such vehicles typically include a shorted diode protection circuit connected in circuit with the field winding of the generator for detecting the occurrence of a short circuit condition. The sudden high current condition in the armature windings will produce a magnetomotive force that almost directly opposes the generator field magnetomotive force thereby tending to de-magnetize or weaken the magnetic field in the stator rotor air gap of the generator. This de-magnetizing magnetomotive force induces extra current in a field winding so that the total flux linkages will remain constant. This extra current will create a reflected voltage on the field winding that can be detected by the shorted diode protection circuit. The shorted diode protection circuit immediately transmits a signal to the system controller to effect a rapid removal of excitation current to the field winding so as to cause the generator power to be rapidly reduced. The effect of the shorted diode protection circuit is to reduce the output current of the generator before the maximum available short circuit current is realized.
In known systems, there has been no convenient method for determining whether or not the shorted diode protection circuit in such a traction vehicle is actually operative, although the circuit can be checked out in a repair facility by either removing the protection circuit from the locomotive or by coupling external power sources to the protection circuit. Accordingly, it would be advantageous to provide a system which could routinely test the operability of the shorted diode protection circuit and advise a locomotive operator whether or not it is safe to operate the locomotive.